For the second day in a row, TriMet has sent out an alert, warning evening MAX commuters to expect delays because of the heat.

Light rail slowdowns happen whenever things get hot in the Portland area. And those TriMet alerts never fail to prompt readers to email Hard Drive with the same question:

Why does MAX run slower in the heat?

It has nothing to to do with trains overheating or potential mechanical problems.

High temperatures can cause light rail power lines to sag and rails to expand. Rail operators monitor conditions in the field as the mercury rises and adjust speeds accordingly.

When the temperature passes 90, TriMet reduces train speeds by 10 mph for all areas with a speed limit of 35 mph and above. If temps reach 100 or higher, they reduce all train speeds to no faster than 35 mph.

The lower speeds help account for any sag in the wire and expansion of the rails, TriMet engineers say.

Unfortunately, for those of you who have a problem with other commuters going shirtless to help them tolerate the Big Hot (only about 72 percent of TriMet buses have air conditioning), TriMet doesn't have a no shoes, no shirt, no service policy.

So, admire that farmer's tan or just close your eyes and try to imagine yourself in a walk-in freezer.